Two stroke cycle engines have been used heretofore mostly in extreme sized applications, being either very small or very large engine applications. The small engine applications are represented by such applications as lawn mowers and motor bikes where the low cost of manufacture is of paramount importance and some inefficiency of operation can be tolerated. In these applications, the bottom of the piston is used as a scavenging blower. The very large marine engine applications have been for use in ore boats, ships, etc., where the large reciprocating mass of the piston and connecting rod that is moved about would cause the four stroke cycle engine operation to be inefficient compared to the two stroke. Also, when these large applications have a need for scavenging and supercharging, large, expensive blowers are required as well as other accessory equipment. Such expense is tolerated because of the large capital investment of the application.
In midsize engine applications, the two stroke cycle engine has not been used widely, except for two stroke diesel truck engines having a four valve exhaust system. In this arrangement the cam operated exhaust ports are operated with an asymmetrical timing relative to BTDC (Before Top Dead Center), but the intake ports are opened and closed symmetrically since they are controlled by the piston operation. During the downstroke the exhaust valves open before the intake ports to assure a blowdown of the relatively high cylinder pressure prior to the scavenging process. On the upstroke the exhaust valves are closed earlier than the intake ports to provide for an increased pressure and charge density in the cylinder prior to the compression event. This timing schedule provides for fairly high specific output. It is not conducive to high fuel efficiency because with a compression ratio, for example, of about 16:1, the expansion ratio would be only 14:1. The fuel efficiency of such engine is related to its expansion ratio.
Another problem that has deterred further development of the two stroke engine for midsize applications occurs at part load where less fuel and less air is required of the engine. If the engine designer were to reduce fuel delivery at part load, the same amount of air would still be pumped in by the constant speed compressor. This lack of proportioning results in misfirings of the engine.